Adenosine is a well-known component of several endogenous molecules (ATP, NAD+, nucleic acids). Besides, it plays an important regulatory role in many physiological processes. The effect of adenosine on heart function was discovered already in 1929. (Drury and Szentgyörgyi, J Physiol 68:213, 1929). The identification of an increasing number of physiological functions mediated by adenosine and the discovery of new adenosine receptor subtypes give possibilities for therapeutic application of specific ligands (Poulse, S. A. and Quinn, R. J. Bioorganic and Medicinal Chemistry 6:619, 1998).
To date, the receptors for adenosine have been classified into three main classes: A1, A2 and A3. The A1 subtype is partly responsible for inhibiting the adenylate cyclase by coupling to Gi membrane protein, partly influences other second messenger systems. The A2 receptor subtype can be subdivided into two further subtypes—A2a and A2b—, which receptors stimulate the adenylate cyclase activity. The sequence of adenosine A3 receptors have been recently identified from rat testis cDNA library. Later it was proved that it corresponds to a novel, functional adenosine receptor. The activation of the A3 receptors is connected also with several second-messenger systems: inhibiting of adenylate cyclase, stimulating phospholipase C and D.
The adenosine receptors are found in several organs and regulate their functions. Both A1 and A2a receptors play important roles in the central nervous system and cardiovascular system. In the CNS, the adenosine inhibits the release of synaptic transmitters which effect is mediated by A1 receptors. In the heart, also the A1 receptors mediate the negative inotropic, chronotropic and dromotropic effects of adenosine. The adenosine A2a receptors, which located relatively in a higher amount in the striatum, display a functional interaction with dopamine receptors in regulating the synaptic transmission. The A2a adenosine receptors on endothelial and smooth muscle cells are responsible for adenosine-induced vasodilation.
On the basis of mRNA identification, the A2b adenosine receptors are widely distributed in different tissues. They have been identified almost in every cell type, but its expression is the highest in the intestine and the bladder. This subtype probably also has important regulatory function in the regulation of the vascular tone and plays a role in the function of mast cells.
Contrary to A1 and A2a receptors, where the tissue distribution was detected on the protein level, the presence of A2b and A3 receptors was detected on the basis of their mRNA level. Expression levels for A3 adenosine receptors are rather low comparing to other subtypes and highly species dependent. A3 adenosine receptors are expressed primarily in the central nervous system, testis, immune system and appear to be involved in the modulation of mediator release from mast cells in immediate hypersensitivity reaction.
For therapeutic use it is essential to ensure that the molecule does not bind, or bind only in the case of very high concentration to the A1, A2a, and A2b sub-types of the adenosine receptor. Our present invention relates to the compounds of the general formula (I) labeled with iodo isotops of mass number 125, and to their salts, solvates and isomers, which have great selectivity for the A3 sub-type of the adenosine receptor.
The [3H]-MRE 3008-F20 adenosine A3 receptor antagonist radioligand is known from the literature. (P. G. Baraldi, Bioorganic and Medicinal Chemistry Letters, 10, 209-211, 2000). Also known is the [3H]PSB-11 A3 receptor antagonist radioligand (C. H. Müller, Bioorganic and Medicinal Chemistry Letters, 12, 501-503, 2002).
Our aim was to prepare A3 radioligands of antagonistic effect labelled with iodine isotop of mass number 125, since these have higher specific activity compared to those labelled with tritium. The goal was to prepare radioligands having strong affinity to the adenosin A3 receptor, but at the same time showing high selectivity within the subtypes, i.e. binding in much higher concentration to the A1, A2a and A2b receptors. A further aim was to have radioligands suitable for the characterisation of the A3 receptor in the different tissues and for the study of the mechanism of action of A3 antagonists.